Sleep Cycle Calculator
Tell it when you want to wake up and get recommended bedtimes for 4 to 6 full 90-minute sleep cycles.
What is it
A full sleep cycle runs about 90 minutes, from light sleep through deep sleep to REM. Waking at the end of a cycle — not in the middle of deep sleep — is what makes you feel rested. This tool counts backward from your wake-up time, subtracts 15 minutes to fall asleep, and lists bedtimes that land you on cycle boundaries.
Notes & fun facts
Ever woken at 6 am feeling like a truck hit you, then another morning the alarm rings and you jump up fresh? The difference is often which stage of sleep the alarm catches. Waking out of deep sleep dumps you into groggy sleep inertia that can last an hour — waking at the end of a cycle barely stings at all.
Origin & history
The 90-minute rhythm was uncovered in the early 1950s by physiologist Nathaniel Kleitman and his student Eugene Aserinsky at the University of Chicago, when they noticed REM sleep recurring in roughly 90-minute loops. Kleitman called it the basic rest-activity cycle; today’s bedtime calculators are a practical offshoot of that discovery.